That is very strange because I am running latest version of Aperture (I assume you are also) and on Maverick 10.9 and don't have this issue. Matter of fact I just attached my iPhone 5 to my iMac to import a video that my daughter took on her iPhone 5 and texted to me. No issues with the thumbnail or playing the video.
Sorry I could not be more helpful, but I don't think that your issue is actually an "aperture issue".
Yes it's definitely a weird issue. And it turns out it also effected even movies taken when our old iPhone 4S and other movies that have been in the library for 5 years without any problems.
I tried to import the photos into the Aperture library through iPhoto since it was the only application that could see the video files. But then it gave me an "unrecognizable format" error. This seems really weird and I'm not sure where the root problem is coming from. I feel like it's missing a codec to support it's own movies.
I managed to get them off the phone and onto a hard drive. When I get info on the videos QuickTime and a bunch of other apps can read the movies. And if I drag and drop them from the hard drive into Aperture it seems to work but not through the import option.